Automobiles and electric trains have a characteristic operation mode of running on inertia (coasting) without acceleration or deceleration of a drive system. A drive system using a synchronous machine generates a no-load induced voltage during such coasting. The no-load induced voltage is full-wave rectified through diodes which are connected in antiparallel to switching circuits that constitute a power conversion apparatus. This increases the direct-current voltage (voltage across the terminals of a capacitor) to regenerate power on the power supply side, and the drive system makes a braking operation as a whole.
As a related technology, Patent Literature 1 listed below discloses a drive unit of a permanent magnet motor for an electric car. The drive unit includes a parallel connection circuit of one-way conducting means and open-close means, inserted in series between the power supply of an inverter and an inverter arm, and drives a permanent magnet synchronous motor through the inverter. When the inverter is not in operation, the open-close means is open. To start up the inverter, the open-close means is kept open while an exciting current is controlled so as to set the terminal voltage of the motor to a predetermined value. When the terminal voltage of the motor reaches the predetermined value, the open-close means is closed and a torque current of the motor is controlled to operate the motor for acceleration or deceleration. To stop the inverter in operation, the exciting current is controlled so as to set the terminal voltage of the motor to a predetermined value while the torque current is reduced to zero. The open-close means is then opened before the exciting current is reduced to stop the operation of the inverter. The technology disclosed in Patent Literature 1 takes account of countermeasures against overvoltage on the capacitor on the power supply side.